The way that we understand human metabolism is through the calories in, calories out model (CICO).

Under this model, changes in your weight are dependent on food intake - when you eat more calories than your body needs, some of this energy is used to build muscle, and some of it is stored as fat. When you eat fewer calories than your body needs, it will either break down muscle or burn fat in order to make up the difference.

Calories in refers to the calories that are being taken in on a daily basis - consumed in your diet. Calories out refers to the calories that are being burned on a daily basis - through your base metabolism (the energy your body needs just to breathe, sleep, maintain homeostasis, digest food, and poop) and through exercise. When there are more calories coming in than going out, you gain weight, and vice versa.

The number of calories in a single pound of bodyweight is roughly 3500 - this number isn't perfect, but it's generally accurate. So for every extra 3500 calories you'll gain about a pound of weight, and for every 3500 fewer calories than you need, you'll lose about a pound of weight.

This model has been validated by years of both practical and theoretical experience. This theory has guided bodybuilders and athletes looking to make favorable changes in body composition as well as general exercisers looking to lose weight. These principles hold true whether or not the exerciser actually believes in them: many diets which claim not to operate on a CICO model are actually operating under its principles all the same.

At the same time, there are many who dispute the accuracy of this way of looking at things. Often, looking to sell new diets or exercise programs, trainers come up with alternative solutions for why people gain or lose weight - presence or absence of the consumption of sugar, fats, carbs, animal products, protein in general, non-paleo foods, and so on. These theories conflict and rarely have much scientific evidence behind them.

Working as a coach and trainer for many years and having a lot of firsthand experience, I’ve seen a lot of these arguments. However, what I find most consistently is that those who claim that the CICO model is inaccurate are often also the people who are most unaware about how the model actually works.

So, I wanted to have the chance to lay out all the variables in play. I’ll also go over some of the most common mistakes people make when interpreting this model.

Calories Out

The “calories out” end of the equation refers to the amount of calories that the body is using per day. I tend to find that people consistently over or underestimate their actual daily caloric consumption. This also usually seems to coincide with whatever narrative you’re trying to push about why you might be failing to achieve the results you want.

Usually, when trying to gain weight, people will overestimate how many calories they’re consuming, and underestimate the calories they’re burning. When trying to lose weight, the problem is the opposite - they overestimate how much they’re burning and underestimate how much they’re eating. In fact, this is generally supported by the scientific research, which suggests that people in general are really bad at estimating their calorie intakes and exercise levels.

Here’s the numerous ways that calories are burned throughout the day, and how to calculate their contributions.

Base Metabolism

The largest contributor is your base metabolism. Again, this is the energy your body needs to carry out vital functions. It’s about how much energy you would burn if you lay in bed all day for a day and did nothing but sleep and eat. Surprise, maintaining your body’s vital functions is surprisingly taxing!

For most people, this can be calculated using a calorie calculator. These equations aren’t perfect, but they give you a rough idea of where your base metabolism should be at. Genetic and hormonal factors can cause small variances in your metabolism, so it’s important to remember that this is just a starting point.

A simple rule of thumb is that your metabolism is roughly 10x your current bodyweight in pounds. I’m a 200lb person, so this means that my metabolism should be roughly 2000/day. The calculator linked above will give you more accurate numbers, but this is a good estimate. Accordingly, my number in the above calculator is closer to 1900 calories.

NEAT

A huge mistake many people make is believing that exercise and base metabolism are the only factors. In fact, there’s another factor called NEAT - non-exercise activity thermogenesis. This is the energy you use when going about your daily life - standing up, sitting down, getting in your car, climbing the stairs, and so on.

NEAT can have a huge impact on your energy needs and can actually contribute a significant amount. If you plug your numbers into the calculator linked above, you’ll see that there are options included for your activity level, the lowest of which is sedentary. This will add in a typical expected number of calories from NEAT per day. What that means is, it assumes you do no serious exercise but are still a regularly active person.

For my numbers, going from calculating my base metabolic rate to calculating my actual calorie needs as a sedentary person causes my number to jump from about 1900 calories to about 2300 calories. That’s a 400 cal/day difference that most people just don’t think about at all - a huge elephant in the room. That’s about a 20% increase, and that’s not even with any exercise added!

Another interesting feature of NEAT is that it can change significantly depending on our dietary intake.

If you’re in a calorie surplus, where you’re taking in more calories than you need, then your body can ramp up NEAT naturally as a way of burning off these excess calories - you fidget more, feel like you have more energy, and will generally burn more NEAT calories throughout the day.

On the reverse end, if you’re in a calorie deficit (trying to lose weight) your body may instinctively reduce NEAT levels as a way of saving energy. A great thing if you’re a caveman trying to survive, but a terrible thing if you’re just looking to knock off some weight in a non-survival scenario. You feel lethargic, you have less energy, and you’re less motivated to expend excess energy during the rest of the day.

In the past, some coaches and trainers have argued for the existence of an effect called “metabolic damage”. The premise of this theory is that your base metabolism can be damaged by long periods of caloric deficit, resulting in an even lower metabolism than should be expected based on your size.

While the evidence is not yet perfect on this subject, it’s also possible that some degree of the effect of “metabolic damage” could be due to natural changes in NEAT during surplus and deficit.

Adjustment for Exercise

Exercise itself burns calories.

It is generally believed that cardio burns more calories than lifting weights. When you hop on a treadmill, it’s easy to count the calories that the machine is giving you and assume that this means you’ve burned a ton of calories - thus, making you think that you should be losing weight pretty quickly!

Unfortunately, the reality is that it’s a lot more confusing than that. The calorie counters on treadmills and other cardio equipment are often off, and sometimes by quite a significant amount. They tend to overestimate the calories you burn consistently for a lot of reasons.

If you’re not using a heart rate monitor in conjunction with the treadmill, then it’s generating caloric burn solely based on tables of data pulled from the average population - in short, it’s providing an estimate based on someone who is roughly your size and age. This is a problem because everyone burns calories at a different rate from exercise.

As you get more skilled with running, for example, you burn fewer and fewer calories when doing the same amount of work because your body gets more efficient at doing it. This means that the more skilled you get, the less accurate these calorie burn estimates are.

A way to improve the accuracy of this estimate is to use the treadmill in conjunction with a heart rate monitor. With the data from a heart rate monitor, the treadmill can use different, more accurate tables based on your actual level of exertion. But even this isn’t perfect.

The most accurate way to measure caloric burn from cardio is to use a metabolic cart. This is a device which measures the amount of oxygen taken in and carbon dioxide breathed out during exercise. Since there’s a direct correlation between how much oxygen is used and how much energy is produced, this can give you very precise estimates of calorie burn. At the same time, these machines are expensive, big, and complex - so they’re rarely seen outside of laboratory or sport-performance settings.

So it’s very hard to actually measure how much energy you’re using during cardio - though you can get a ballpark estimate by using a good heart rate monitor watch or something similar. What about lifting weights?

Unfortunately, it’s even harder to estimate caloric burn when lifting weights. The equations that heart rate monitors use to estimate your caloric burn during cardio are not effective at estimating caloric burn during lifting weights because the two activities are so different.

Cardio relies on steady elevations of heart rates while weight lifting relies on spiking your heart rate during the set and then waiting for that heart rate to come back down.

This means that heart rate monitors often misjudge the amount of energy burned during lifting activities. Again, there’s no easy solution to this - there’s not really an accurate way to estimate how much energy you’ve burned short of using a metabolic cart. There are no existing commercial products that I know of that have yet found out a way to effectively figure out energy burn from exercise - and many of them still claim they can, even though they rely on cardio equations for calculating weight lifting caloric burn.

One way to estimate your calorie burn from activity levels is to use activity adjustments in the calorie calculator we used above. In my case, switching the activity variable from sedentary to lightly active estimates that I burn about an additional 400 cal/day on top of the existing numbers for NEAT and base metabolism.

In general, you can expect to burn a few hundred calories in an hour long workout, but you can’t be 100% sure exactly what that number will be. Heavier exercisers will burn more calories and lighter exercisers will burn fewer, and more skilled exercisers will be more efficient at exercising and thus burn fewer calories at any level.

Recovery from Exercise

Another aspect of calorie burn is through recovery from exercise. In the time immediately following a workout, your energy stores are depleted and your body is working overtime to replenish them. In the days following a workout, your heart rate may remain elevated, leading to more calorie burn. Additionally, the process of remodelling tissue, rebuilding muscle, and recovering from an intense workout certainly requires energy. This effect of requiring more energy in the hours and days following a workout is called the “post exercise oxygen consumption effect” or EPOC.

EPOC is not likely to be a significant contributor to weight loss since the effect is relatively small. However, when it comes to gaining weight, many lifters are working out way too frequently, keeping their metabolism high and thus sabotaging their efforts to put on weight. It's also simply another number that many people overlook.

This number is normally included in calculators as part of the activity adjustment - so the 400cal/day from the adjustment for exercise above already also includes expected calories for EPOC and general recovery.

Calories In

The “calories in” end of the equation refers to the amount of incoming calories. This is a much generally simpler equation, and one that we can understand a bit more fully.

Calories in come from all of the food we eat and the liquids we drink. The biggest consideration here is hunger: are you feeling hungry, or full, from a meal?

The body’s hunger response is fine-tuned to provide us with the right amount of energy that we need throughout the day. Normally, it’s quite good at figuring this out, so it will cut off hunger when it recognizes that you have enough energy. But there are numerous ways to trick the hunger response, leading to confusion about how much food you’re actually eating.

In particular, calorically dense, hyperpalatable (delicious) foods make it easy to overeat on calories without being particularly filling. There’s no magic here, no evil foods, and no magic bullet - it’s just that we tend to overeat these foods because we enjoy the taste and/or because they’re not too filling relative to their actual calorie content.

In contrast, minimally processed foods high in protein and fiber (your meats and veggies) tend to be the most filling relative to their actual caloric content. These make it much easier to train your hunger response to be more accurate.

For this reason, it’s often very easy for less knowledgeable trainers to make the argument that calories don’t matter - because you’re eating fewer calories but still feeling full, you might get the feeling that you're eating more calories when you're actually eating less!

The biggest mistake people make with their diet is not tracking in any meaningful way. I can’t tell you how many people swear that they should be losing or gaining weight, but are only doing so off of a vague memory of what they ate and how hungry they were. Without actually tracking calories, you can’t really know what your caloric intake looks like in any meaningful way, because again we’re pretty bad at estimating. If you’re not tracking, you don’t actually have any clue how much you’re eating or whether or not you should be gaining or losing weight.

This doesn’t mean that you have to measure and weigh out every single gram of food for perfect precision. This is an extremely taxing and difficult process which can take a great deal of time, and which most people aren’t ready for. It may become necessary when working with someone like a bodybuilder, who is highly motivated and has great demands of their physique, but it’s much less useful for the common person. Instead, eating strategies and other less precise but also less difficult methods of tracking may be more useful.

Counting calories is an art in and of itself, and it's hard to do so with a great deal of accuracy without also a great deal of practice. This skill may be too much for some people, and there's nothing wrong with that. But it's also something that people can often mess up - if you don't weigh out your food, or work off of the wrong assumptions about the calorie composition of a certain food, you can get inaccurate numbers that aren't indicative of how much you're actually eating.

Water Weight

One confounding factor when it comes to losing or gaining weight is the problem of water weight. Water weight accounts for a large chunk of your overall weight, since human beings are made up of a lot of water. We need it to survive and to keep our internal systems running.

Physically, it’s possible to lose a significant amount of water weight in a relatively short period of time without actually causing any changes in lean or fat tissue mass - the stuff we normally think of as what changes when we gain or lose weight.

This strategy is often used by powerlifters, fighters, and others who compete in weight-class-based competitions. By rapidly dehydrating themselves, they lose enough weight to fit into a lower weight class, regain the weight quickly through sports drinks, and then compete fully rehydrated. heavier than they would be otherwise.

Exercise can also cause a great deal of loss of water weight. I remember that I used to experience 5lb water losses during intense (1-2hr long) cardio workouts. Of course, this weight is immediately put back on when you start to drink water to replenish what was lost, so this certainly isn’t any kind of long term weight loss strategy. It also explains why things like weight loss wraps and hot yoga, which induce heavy sweating, initially "seem" to work - but have no effect in the long term.

On a smaller basis, changes in water weight can mask changes in lean or fat mass. If you’ve lost a pound of fat but you’ve temporarily gained two pounds of water weight, you might think that you’ve gained weight instead of losing it.

One method of avoiding this issue is by simply weighing yourself early in the morning every day (this method is recommended). In this way, water weight should have minimal impact on your weight. Consistent measurements over time so you have a lot of data points to compare to is also a necessity. If you weigh yourself a lot, you'll be able to chart actual changes in your average weight over time instead of focusing on minor fluctuations in your water weight.

Fluctuations in water weight can cause problems in interpreting changes in your overall weight. This isn’t directly related to your metabolism, but can cause problems in interpreting weight gain or loss, leading you to potentially believe that calories don’t matter. If you ate 3500 fewer calories this week but water weight changes cover this up, you might imagine that these calories didn't mean anything.

Another Solution

When it comes to gaining or losing weight, the biggest problem is that most of the numbers involved (calories in, NEAT, base metabolism, exercise calories burnt, post exercise calories burnt) are all quite fuzzy. This doesn’t mean that they aren’t real and don’t exist, just that some degree of inaccuracy should always be expected.

One way to calculate your actual metabolism with great accuracy involves weighing yourself daily. If you add these seven numbers together and average them, you get your average weight for the weak. If you repeat this again next week, you have your average weight for the next week. Now, you find the difference between these two numbers (in pounds) and multiply by 3500 - roughly the number of calories in a pound of bodyweight. This represents your overall change in calories from the first week to the second week, positive or negative.

This number can then be compared up against your expected weight gain or loss for that week. If you’re trying to lose weight and eat (what you think to be) 500 calories less than your metabolism per day (a total of about -3500 calories) but found that you have lost less than a pound of weight, what that means is that your metabolism was actually a bit higher than you thought, to the tune of the difference between your expected weight and actual weight. If you lost more than the expected pound of weight, that means that your metabolism was actually higher than you thought.

This approach isn’t perfect either. The biggest problem is that it requires very accurate reporting of calories eaten as well as regularly weighing yourself. If these numbers are inaccurate, then the equation goes bonkers. But it’s also a way of very accurately calculating your actual metabolism without needing to worry about all the complex bits and pieces that go into it.

Still, this is a bit complicated, even if it’s completely accurate. Luckily, there’s a product out there that does this math for you: Greg Nuckols’ Self Correcting Macro Tracker, which operates on this principle, is included in his Training Toolkit product for a cheap $10. If it sounds like something you’re into, this product is absolutely magical in that it handles all these complex equations pretty easily and accurately. Still, I highly recommend being sure you’re prepared to jump into that degree of tracking, since it can be hard to do.

CICO Works!

The reality is that CICO as a theory is remarkably accurate for describing the process with which your body uses to gain or lose weight. However, CICO is often misinterpreted and misunderstood by its critics, who claim that it’s impossible that it should work generally simply because they are ignorant of all the complex working parts that go into it.

Your metabolism isn’t a set number, but is instead a highly fluid collection of variables, all of which can change relatively quickly. Even an equation as accurate as the one explored above, while extremely accurate, also involves a great deal of possible error and would result in calculations of regular changes in your metabolism from week to week.

And that’s the thing - that your metabolism changes regularly. It’s also often much higher or lower than people tend to think, depending on what their goals are. Weight loss exercisers often think their metabolism must be much higher than it actually is, and weight gain exercisers often think their metabolism must be much lower than it actually is.

Without careful tracking, it is very difficult to really have any clue what your metabolism is. Claiming that CICO isn’t accurate or real often relies on exactly this position, from which anything can be argued because there is little evidence.

Takeaway Points

The calories in, calories out model posits that humans gain weight due to excess calories, and lose weight when they consume fewer calories than they need per day. The rough number is 3500 calories per pound.

Base metabolism is the number of calories your body burns per day simply on vital functions. This is generally related to body size and muscle to fat ratios.

Non exercise activity thermogenesis (NEAT) is the number of calories burned daily on non-exercise activities. This increases as weight increases, and can adapt to suit the body's current caloric intake levels.

Exercise burns calories. However, it's harder to judge exactly how many calories it burns than most people think. Metabolic calculators generally solve for this by adding an adjustment for activity levels, but it's not perfect.

Recovery from exercise also takes calories. However, these calories are generally factored into the activity level adjustment above.

Your calories in are determined by how much you eat. Without accurate tracking, you cannot know exactly how many calories you're eating from hunger levels alone, which aren't an accurate measurement.

Water weight can change, causing your weight to change without any changes in the underlying muscle or fat mass. This can hide actual changes in body size in the short term.

Strategies which cause a great deal of sweating will cause short term weight loss, but this vanishes when you rehydrate.

One way to calculate your actual metabolism with a great deal of accuracy is to compare your week over week change in average weight to your expected change in average weight for that week, allowing you to adjust for your actual metabolism.

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